In packet transmission systems such as, for example, WLAN, the payload data of a data packet are preceded by a known preamble in order to support the acquisition at the receiver end. The data packets have different lengths and can arrive at the receiver at times which are not or only inaccurately known. For this reason, the receiver must first perform an acquisition in which, among other things, the preamble, and thus the reception of a data packet, are detected and then the accurate position of the preamble in time and thus the position of the subsequent payload data in time are determined.
In preamble detection and frame synchronization (time synchronization), the uncertainty about the time of arrival of a data packet at the receiver end and the characteristics of the transmission channel present problems. Furthermore, the transmission protocol often requires fast preamble detection such as, for example, in WLAN 802.11 a/g standard in which the preamble detection must have taken place within 4 μs after the beginning of the data packet.
Furthermore, preamble detection and frame synchronization are rendered more difficult by the situations listed below:                The multipath channel h=[h(−L1) . . . h(0) . . . h(+L2)] with the time-variant channel coefficients h(i) is unknown. Furthermore, the multipath profile E[|h (i)|2] and the length L=L1+L2 of the multipath channel are also unknown.        The payload and noise signal levels and, as a consequence, the signal-to-noise ratio are also unknown.        The front end exerts unknown influences at the beginning of the data packet reception. In particular, the RSSI (radio signal strength indicator), AGC (automatic gain control) and VCO (voltage controlled oscillator) units cause signal level, frequency and phase transients and an unknown frequency offset, as a result of which a part of the preamble cannot be detected and the first samples can be greatly distorted.        The preamble has a disadvantageous structure which impairs the determination of the accurate position of the data packet in time. For example, the preamble in the WLAN 11a standard has the structure [B B B B B B B B B B C1 C2 C1 C2 C1]. This preamble contains 10 B segments with a respective duration of 0.8 μs and a length of 16 preamble symbols and 5 C segments with a respective duration of 1.6 μs and a length of 32 preamble symbols. The beginning of the preamble, and thus also the beginning of the payload data, can only be determined by searching for the transition from the B segments to the C segments.        
Preamble detection and frame synchronization has hitherto been based on the autocorrelation of periodic signal sections. In this process, use is made of the fact that, although periodic signals are distorted by the aforementioned influences, they remain periodic at the transitions apart from phase rotations and transients.
During the autocorrelation of two successive signal sections with a known period length, a flat peak of the metrics is obtained at the output of the correlator whenever the correlator exclusively correlates samples of the wanted preamble with one another. To illustrate this situation, FIG. 1 shows a data packet with a preamble consisting of B, C1, and C2 segments. The metrics M resulting from the autocorrelation are also shown.
After the autocorrelation of the samples, the resultant signal is usually subjected to postprocessing such as, for example, threshold detection and consistency checks in order to increase the probability of detection and, at the same time, to keep down the false alarm rate.
One disadvantage of preamble detection by autocorrelation is a high latency since the peak of the metrics at the correlator output is only reached after two period lengths. A further disadvantage of the autocorrelation is the fact that frame synchronization is only inaccurate since the metrics can recognize only soft transition at the B-C transition. Furthermore, the autocorrelation is susceptible to interference from unwanted signals which have a similar period length to the wanted preamble.
Hitherto, no devices based on the principle of optimum detection of a known signal in the noise when the signal has passed through an unknown multipath fading channel has been used for preamble detection and frame synchronization. Such optimum detectors are described in the book “Statistical Signal Processing—Vol. II: Detection Theory” by S. M. Kay, published by Prentice-Hall, 1998.
An optimum detector consists of a RAKE receiver having a number of RAKE fingers. Each RAKE finger determines the energy proportion of a transmission path. For this purpose, each RAKE finger contains a cross correlator and a noncoherent detector. The RAKE components are weighted, added together and then supplied to a threshold detector. As a result, an optimum detector in each case calculates metrics for the hypothesis according to which the wanted signal was received, and a hypothesis for the fact that the wanted signal was not received. Following this, a threshold decision is carried out. However, optimum detection is only possible theoretically under the following boundary conditions:                The multipath profile E[|h(i)|2], the length L and the noise level are known.        The front end does not exert any influences on the preamble detection. In particular, there are no phase transients due to VCO settling and frequency offset.        The received signal is available over the entire length of the wanted preamble.        The time pattern of the wanted preamble is known, i.e. certain starting times are predetermined such as, for example, in the case of time slots in TDMA-based mobile radio systems.        
The boundary conditions listed above are not met due to the aggravating situations listed above and the demand for fast preamble detection. In detail, the multipath profile and the signal-to-noise ratio are not precisely known, the front end generates strong phase transients, the available signal spacing for the fast preamble detection is only short and the times at which data packets arrive are completely unknown. For these reasons, optimum detectors have hitherto not been used for preamble detection and frame synchronization.